1. Field of the Invention
The present invention relates to a power amplifier, and more particularly, to an adaptive predistortion linearizer capable of appropriately compensating changes in power amplifier""s nonlinear characteristics.
2. Background of the Related Art
A power amplifier is used to increase the power of an input signal. An ideal power amplifier only increases gain (size) without otherwise distorting an input signal. However, since a power amplifier generally includes active devices having nonlinear characteristics, the output of the power amplifier inevitably contains a distortion component.
Accordingly, a variety of techniques and algorithms for improving the power amplifier""s nonlinear characteristics have been proposed. Examples of these include predistortion, polar envelope correction, bias compensation, feedforward, etc. Of these examples, the predistortion method has a simple structure and an excellent efficiency, as compared to the feedforward method, and unlike the polar envelope method, has no limitations on bandwidth. In addition, the predistortion method is widely used in mobile communication base stations and the like, as a linearization method is more adaptable than the bias compensation method.
FIG. 1 illustrates a related art predistortion-type linearizer, including a predistorter 1 and a power amplifier 2. The predistorter 1 includes a FET (field effect transistor) or a diode.
A gain (size) and phase of the power amplifier 2 changes according to an input signal. In other words, an amplitude and phase of the output signal is nonlinearly outputted according to the size of the input signal This is referred to as AM-AM and AM-PM distortion. Thus, the output of the power amplifier contains a distortion component in addition to the input signal, due to the AM-AM and AM-PM distortion. This is generally called Inter-Modulation Distortion (IMD). The IMD due to the AM-AM and AM-PM distortion can cause a spectral regrowth in a digital communication system, thereby causing degradation of the communication system.
Therefore, as illustrated in FIG. 1, the predistorter 1, having the inverse/reverse characteristics of the AM-AM and AM-PM characteristics, is connected to the front end of the power amplifier 2 to remove the AM-AM and AM-PM distortion of the power amplifier 2. That is, if the gain and phase characteristics of the power amplifier 2 based on the size of an input signal are the same as shown in FIG. 2B, then the predistorter""s characteristics are made to have the same characteristics as shown in FIG. 2A. This results in an output of the power amplifier 2 shown in FIG. 2C, the IMD due to the AM-AM and AM-PM distortion.
As described above, the predistorter in the related art predistortion-type linearizer has various problems. For example, it generates the reverse characteristics of the nonlinear characteristics of the power amplifier by using the nonlinear characteristics of the FET or DIODE. However, this method requires much time and labor since it must be implemented so that the nonlinear characteristics of the FET or DIODE are exactly the reverse of the power amplifier""s characteristics.
In addition, the power amplifier""s nonlinear characteristics (AM-AM, AM-PM) typically change over time or due to external environment (temperature, bias). Hence, in order to maintain the linearity of the power amplifier, the predistorter""s non-linear characteristics (AM-AM, AM-PM) must be changed according to changes in the power amplifier""s nonlinear characteristics. However, the related art predistorter""s non-linear characteristics cannot effectively respond to the changes in the power amplifier""s nonlinear characteristics. Thus, the linearity of the entire power amplifier is reduced.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
Another object of the present invention is to provide an adaptive predistortion that substantially obviate problems caused by limitations and disadvantages in the related art.
Another object of the present invention is to provide an adaptive predistortion linearizer capable of compensating changes in power amplifier""s nonlinear characteristics, whether based on the lapse of time, an external environment or otherwise.
Another object of the present invention is to provide an adaptive predistortion linearizer capable of implementing the reverse characteristics of power amplifier""s nonlinear characteristics in a simple manner.
To achieve at least these objects, in whole or in part, there is provided an adaptive predistortion linearizer according to the present invention, which includes: a control unit for controlling a gain and phase of an input signal so that they are the reverse of the power amplifier""s nonlinear characteristics; and a unit for compensating changes in the power amplifier""s distortion characteristics according to the lapse of time or an external environment, upon receipt of the output of the control unit.
In addition, to achieve at least these objects in whole or in part, there is provided an adaptive predistortion linearizer, which includes a divider for dividing an input signal into first through third paths; a fast gain/phase adjuster for adjusting a gain and phase of the input signal divided into the first path; a slow gain/phase adjuster for adjusting a gain and phase of an output signal of the fast gain/phase adjuster; a power amplifier for amplifying an output signal of the slow gain/phase adjuster to output the same; a fast gain/phase controller for outputting a first control signal according to the envelope of the input signal divided into the second path; and a slow gain/phase controller for outputting a second control signal by comparing the input signal divided into the third path with the envelope of the output signal of the power amplifier.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.